A video image is constructed from a rectangular array of coloured dots. On a raster display device, each dot consists of a red, a green and a blue phosphor and the relative brightness of these three phosphors (colour components) determines the colour perceived by the viewer. In a standard graphics computer, the information comprising the video image is stored in a framestore. This is a segment of random access dual ported memory in which the colour of each dot in the video image is represented by one or more numbers. Each dot is known as a picture element or pixel. Several formats exist for encoding the picture colour information, the most common one being the Red Green Blue format (RGB) whereby one number per pixel is used to represent the brightness of each of the red, green and blue colour components. In existing systems, the image is then displayed by scanning the contents of the framestore with a direct video readout and lighting the screen phosphors accordingly.
The original images which are to be treated may be a video image itself or it may be derived from an analogue form such as photographic film, graphics or a conventional form of artistic representation. It may contain any number of pixels in both horizontal and vertical dimensions.
Usually, each component in the framestore is represented at each pixel by a value ranging from 0 to 255:0 corresponding to darkness and 255 to full brightness for that component. The number is representable in computer memory by one byte of information (8 binary bits). Hence a full colour image comprising red, green and blue components requires storage of 3 bytes (24 bits) per pixel. This is the familiar 24 bit framestore. The invention is applicable to images and framestores of any size. 8 bits and 24 bits are only quoted as examples.
It is known for a video image to be overlaid on a second video image by supplementing the 6 values corresponding to the 3 colour components of the two images with a further value, also ranging from 0 to 255, representing the alpha value. This value (sometimes known as the mask, stencil or blend value) governs the extent to which the overlying image replaces by its own colour values the colour values of the corresponding point of the image below. If it is desired that the alpha value is displayed superimposed over the image itself, a 32 bit framestore is required rather than a 24 bit framestore.
If a combined image comprises 2 layers then the arrangements of the prior art require further specialist hardware in the form of at least 2 24 bit framestores and a further store for the alpha value of each pixel. If the image comprises more than 2 layers, it follows that an additional framestore would be required for each additional layer. Such use of multiple framestores for multi-layered images is both complex and expensive in terms of hardware requirements. It is also only possible to edit one layer of the image at a time.